1. Field of the Invention
The present invention relates to an exhaust purification device for an engine, and more specifically, to an exhaust purification device in which a swirling flow is generated in engine exhaust gas, and an additive sprayed into the exhaust gas is supplied to a catalyst device located downstream together with the exhaust gas.
2. Description of the Related Art
In respect of an exhaust purification device that uses an additive to defuse toxic substances contained in the exhaust gas and purify the exhaust gas, it is important to mix the sprayed additive well with the exhaust gas, making use of the swirling flow, and to uniformly diffuse and atomize the mixed additive in the exhaust gas. If these conditions are satisfied, the additive is substantially uniformly supplied throughout the catalyst device, and the high purification performance of the catalyst device is achieved. Various techniques have been designed in consideration of such demand. For instance, JP 2006-183509 (hereinafter, referred to as Patent Document 1) discloses an exhaust purification device that carries out the techniques.
In the exhaust purification device disclosed in Patent Document 1, a swirling flow is generated in exhaust gas by a swirl generator that is disposed in the large-diameter portion of an exhaust path. In the process where the generated swirling flow passes through a taper portion toward a small-diameter portion, the swirling flow is increased in swirling velocity by being reduced in swirl radius by degree. This promotes the mixture of the additive with the exhaust gas and advances the diffusion and atomization of the additive in the exhaust gas. In the exhaust purification device of Patent Document 1, the exhaust path includes a portion with a diameter tapering toward the small-diameter portion and a portion extending from the small-diameter portion and being increased in diameter with steps to be connected to a catalyst device located downstream. There is another exhaust purification device in which an exhaust path has a diameter that is reduced in one portion and then gradually increased in another, and is connected to a catalyst device as shown in FIG. 2.
In the example illustrated in FIG. 2, there is provided an SCR catalyst 116 (selective reduction-type NOx catalyst) for removing NOx (nitrogen oxide), which functions as a catalyst device. A venturi-shaped mixing chamber 113 formed of a taper portion 113a, a constricted portion 113b and a flared portion 113c is interposed in an exhaust pipe 110. In the process where the exhaust gas flows through the mixing chamber 113, a swirling flow that is generated by a fin device 118 is gradually increased in swirling velocity within the taper portion 113a along with the decrease of the swirl radius. The swirling flow is then guided to the SCR catalyst 116 while being gradually increased in swirl radius within the flared portion 113c. In light of the knowledge that an additive is preferably sprayed at a position where an exhaust flow rate is high so that the additive sprayed from an injection nozzle 119 may be well mixed with exhaust gas and that the additive may be diffused and atomized uniformly in the exhaust gas, the exhaust purification device of the above-mentioned type is provided with an injection nozzle 119 that is disposed in between the taper portion 113a and the flared portion 113c of the mixing chamber 113, that is, in the constricted portion 113b (minimum-diameter position) where the exhaust flow rate reaches its highest value.
In many cases, however, distance L′ between the fin device 118 and the SCR catalyst 116 cannot be sufficiently ensured due to constraints associated with the configuration of the exhaust pipe 110 and the like. For this reason, in the exhaust purification device of Prior Art shown in FIG. 2, either one of distance Lf′ between the fin device 118 and the constricted portion 113b of the mixing chamber 113 or distance Ln′ between the injection nozzle 119 and the SCR catalyst 116 has to be set at a smaller value than an optimum value.
In the taper portion 113a of the mixing chamber 113, the mixture of the exhaust gas with the additive is facilitated by gradually reducing the swirl radius of the swirling flow generated in the exhaust gas and increasing the swirling velocity at the same time. If the distance Lf′ between the fin device 118 and the constricted portion 113b is shortened, the swirl radius of the swirling flow is drastically reduced, and kinetic energy is lost. Therefore, the swirling velocity cannot be adequately increased. This causes an insufficient mixture of the additive and the exhaust gas. The additive sprayed from the injection nozzle 119 is diffused and atomized in the exhaust gas while being transferred in the downstream direction. If the distance Ln′ between the injection nozzle 119 and the SCR catalyst 116 is shortened, it becomes impossible to ensure sufficient time required for the diffusion and atomization of the additive. This incurs insufficient diffusion and atomization of the additive.
The exhaust purification performance of the SCR catalyst 116 is delivered to the fullest extent when the good mixture of the additive and the exhaust gas and the uniform diffusion and atomization of the additive in the exhaust gas are both accomplished. In this view, the above-mentioned Prior Art has room for improvement in terms of exhaust purification performance of the SCR catalyst 116.